Compile for Mobile targets

[tiero]

To allow Mobile application to handle Elements data structures gomobile bind seems a great fit in order to compile for native targets (Android and iOS) go-elements.

The caveat here is that we have only a subset of Go types which are currently supported and moreover return variables should be at maximum 2 where the second MIST be error

Binding all the packages we have in go-elements seems not feasible as per experiment here. Also it's impossible to bind address package for iOS.

[tiero]

Proposal A

Create dumb bindings for go-elements to support go-mobile

[tiero]

Proposal B

Replicate what we have done for JavaScript with ldk in Go, having all the abstractions for Identity and Wallet, and creating a more high level API for mobile wallet based on go-elements. Tentative repository has been taken at ionio